a) Field of the Invention
The invention relates to a laser scanning microscope with AOTF.
b) Description of the Related Art
It is already known to combine lasers of different wavelengths in a laser scanning microscope by means of a beam splitter and to couple the lasers into a light-conducting fiber by means of an AOTF (acousto-optic tunable filter) with a grating which is realized by varying the refractive index and whose grating constant can be varied corresponding to its high-frequency driving. The wavelength, which is diffracted in the first order by the AOTF, and the intensity are adjusted by controlling the AOTF; the intensity is adjusted by the amplitude of the sound wave and the wavelength is adjusted by the frequency of the sound wave.
The use of acousto-optics for line selection and attenuation of laser lines in a modular construction has the disadvantage that the transmission characteristics of the acousto-optic unit is highly dependent on temperature. This can be explained by the fact that a change in temperature brings about a change in the velocity of sound in the crystal material which makes itself apparent indirectly in a deviation from the optimum frequency and accordingly in a decrease in the diffraction efficiency. A frequency shift of approximately 16 KHz/° C. was determined. This leads to intensity losses and to possible intensity modulations in the scanned image.
For example, when the AOTF was adapted to 21° C., the transmitted output drops to approximately 5% of the initial value with a change in temperature from 21° C. to 35° C. without corrective steps. Even with a change in temperature of only 4° C., an output drop of about 50% occurs.
An acceptable performance of the laser scanning microscope can only be ensured when the temperature fluctuations can be limited to +/−1° C. However, this is hardly possible in practice due to the many and various possible uses of the laser scanning microscope. Moreover, room temperatures of greater than 30° C. are quickly reached in many laboratories.